归并排序

import java.io.*;

class MergeSort {
  // Merges two subarrays of arr[].
  // First subarray is arr[l..m]
  // Second subarray is arr[m+1..r]
  void merge(int arr[], int l, int m, int r) {
    // Find sizes of two subarrays to be merged
    int n1 = m - l + 1;
    int n2 = r - m;

    // Create temp arrays
    int L[] = new int[n1];
    int R[] = new int[n2];

    // Copy data to temp arrays
    for (int i = 0; i < n1; ++i)
      L[i] = arr[l + i];
    for (int j = 0; j < n2; ++j)
      R[j] = arr[m + 1 + j];

    // Merge the temp arrays

    // Initial indices of first and second subarrays
    int i = 0, j = 0;

    // Initial index of merged subarray array
    int k = l;
    while (i < n1 && j < n2) {
      if (L[i] <= R[j]) {
        arr[k] = L[i];i++;
      }
      else {
        arr[k] = R[j];j++;
      }
      k++;
    }

    // Copy remaining elements of L[] if any
    while (i < n1) {
        arr[k] = L[i]; i++; k++;
    }

    // Copy remaining elements of R[] if any
    while (j < n2) {
        arr[k] = R[j]; j++; k++;
    }
  }

  // Main function that sorts arr[l..r] using
  // merge()
  void sort(int arr[], int l, int r) {
    if (l < r) {
      // Find the middle point
      int m = l + (r - l) / 2;

      // Sort first and second halves
      sort(arr, l, m);
      sort(arr, m + 1, r);

      // Merge the sorted halves
      merge(arr, l, m, r);
    }
  }

  // A utility function to print array of size n
  static void printArray(int arr[]) {
    int n = arr.length;
    for (int i = 0; i < n; ++i)
        System.out.print(arr[i] + " ");
    System.out.println();
  }

  // Driver code
  public static void main(String args[]) {
    int arr[] = { 12, 11, 13, 5, 6, 7 };

    System.out.println("Given array is");
    printArray(arr);

    MergeSort ob = new MergeSort();
    ob.sort(arr, 0, arr.length - 1);

    System.out.println("\nSorted array is");
    printArray(arr);
  }
}